1. Field of the Invention
The present invention relates to light diffusion sheets having a function to diffuse transmitted rays of light, which are suitable for a backlight of a liquid crystal display device in particular, and backlight units in which this light diffusion sheet is used.
2. Description of the Related Art
Liquid crystal display devices in widespread use have been in a backlight system where light emission is executed by irradiating onto a liquid crystal layer from the back face. In such a type of a display device, a backlight unit which is an edge light type, an immediate beneath type or the like is provided to an under face side of the liquid crystal layer. Such a backlight unit 20 of an edge light type is generally equipped with a rod-shaped lamp 21 for use as a light source, an optical waveguide plate 22 having a square plate shape disposed so that the edge thereof abuts along the lamp 21, a light diffusion sheet 23 disposed to the front face side of the optical waveguide plate 22, and a prism sheet 24 disposed to the front face side of the light diffusion sheet 23, essentially as shown in FIG. 3(a).
Referring to functions of this backlight unit 20, rays of incident light from the lamp 21 to the optical waveguide plate 22 are first reflected on reflection dots or a reflection sheet (not shown in the Figure) of the back face of the waveguide plate 22, and exit from the front face of the waveguide plate 22. The rays of light exited from the waveguide plate 22 enter into the light diffusion sheet 23, then are diffused by the light diffusion sheet 23 and exit from the front face of the light diffusion sheet 23. Thereafter, the rays of light exited from the light diffusion sheet 23 enter into a prism sheet 24, and exit as rays of light having a distribution representing a peak in a direction along a substantially normal line via a prism part 24a formed on the front face of the prism sheet 45.
Accordingly, the rays of light that exited from the lamp 21 are diffused by the light diffusion sheet 24, and refracted by the prism sheet 24 so that they represent a peak in a direction along the substantially normal line, and illuminate the entire face of the liquid crystal layer on the front face side (not shown in the Figure). Meanwhile, although not shown in the Figure, an additional light diffusion sheet is disposed to the front face side of the prism sheet 24 for the purpose of: relaxation of light condensing properties of the prism sheet 24 as described above; protection of the prism part 24a; or prevention of the sticking between the liquid crystal panel such as a polarization plate and the prism sheet 24.
The light diffusion sheet 23 to be disposed to the backlight unit 20 generally has a transparent substrate sheet 26 made of a synthetic resin, a light diffusion layer 27 overlaid on the front face of the substrate sheet 26, and a sticking preventive layer 28 overlaid on the back face of the substrate sheet 26 (see, for example, JP-A-7-5305, JP-A-2000-89007) as shown in FIG. 3(b). In general, this light diffusion layer 27 includes a light diffusing agent 30 in a binder 29, and thus a function to diffuse transmitted rays of light is ensured by the light diffusing agent 30. Further, the sticking preventive layer 28 includes a small amount of dispersed beads 32 in a binder 31, and has a structure with lower parts of these beads 32 projecting from the back face of the binder 31. Accordingly, disadvantages of the occurrence of interference fringes through close contact of the back face of the light diffusion sheet 23 with the front face of the waveguide plate 22 are prohibited.
In the meantime, since the light diffusion sheet 23 is generally formed from a synthetic resin, there exist disadvantages of being susceptive to deformation and discoloration (yellowing and the like) by heat, ultraviolet ray and the like. On the other hand, the lamp 21, which is a source for generating rays of light generates heat in concurrence with light emission. In general, a proximal part to the lamp 21 out of the light diffusion sheet 23 is exposed to the temperature of around 80° C. to 90° C. Thus, the light diffusion sheet 23 partially bends upon heat deformation. As a result, there arise problems of occurrence of lack in uniformity of the brightness of a display window.
Therefore, techniques have been developed contemplating the improvement of heat resistance by including a dispersed fine inorganic filler within a binder 29 of a light diffusion layer 27 in a light diffusion sheet 23 (see, JP-A-2000-89007), however, they involve problems of: (a) inferior dispersibility of the fine inorganic filler resulting in impossible achievement of sufficient heat resistance; and (b) insufficient close contact between the fine inorganic filler and the binder 29 to cause fine gaps at boundary surfaces of both of them, resulting in reduction of the strength and transmittability of rays of light.